1. Field of the Invention
The invention relates to a video signal processing apparatus which is provided for a digital video decoder for decoding digital composite video signals in order to reconstruct an image from the digital composite video signals and, more particularly, to a video signal processing apparatus for suppressing a fluctuation of the number of sampling data of the digital composite video signals.
2. Related Background Art
Hitherto, an analog composite video signal having information of one scanning line of an image is sampled at a predetermined sampling period, so that resultant digital composite video signals are sequentially supplied to a digital video decoder. Each of the analog composite video signals constructed on a scanning line unit basis includes: sync signals such as horizontal sync signal and vertical sync signal; a luminance signal and a chroma signal of one scanning line; and the like.
When a fluctuation occurs in the period of each of the analog signals due to a change of a radio wave propagating situation of each of the analog composite video signals or the like, a fluctuation occurs in the number of sampling data in each digital composite video signal corresponding to each analog signal in association with the fluctuation of the period. If the digital composite signal in which the number of sampling data, that is, the number of pixel data for one scanning line does not coincide with a predetermined reference value continuously, for example, for one field, it becomes a cause of disturbance of a reproduced video image.
Hitherto, an adjusting circuit is provided to suppress the disturbance of the image due to the fluctuation of the number of pixel data. According to the adjusting circuit, when the disturbance as mentioned above occurs in the image, an operation signal for a partial adding/deleting process of the digital signals can be manually properly supplied to the adjusting circuit.
By the operation of the adjusting circuit based on the operation signal, a predetermined number of digital data of a pixel data unit can be added or deleted to/from the position corresponding to a blanking portion of the analog signal in each digital composite video signal in accordance with the fluctuation of the number of pixel data, so that the fluctuation of the number of pixel data in the digital signals can be corrected.
It is, however, not easy that each time the disturbance starts to occur in the image, a chain of adjusting operations including a discrimination about whether the adjusting operation should be performed or not is executed by the manual operation as mentioned above. It is, therefore, demanded to automatize the chain of adjusting operations mentioned above.
On the other hand, there are: an adjusting mode in which the digital data is partially added or deleted to/from each digital composite video signal in order to correct the number of pixel data of the received digital composite video signals; and a non-adjusting mode in which such an adjustment is unnecessary for the digital video signals because of the coincidence between the numbers of pixel data and the reference value. If a mode switching is caused between both the adjusting mode and the non-adjusting mode due to the frequent switching operations between the operation and the non-operation of the adjusting circuit, contrarily, the reproduced image is liable to become unstable.
Therefore, even if a difference occurs temporarily between the number of pixel data for one scanning line and the reference value, when such a difference is very small, it is desirable not to perform the adjusting operation upon elimination of an unstable factor due to the frequent mode switching operations.
When the difference due to the dissidence mentioned above is large enough to exceed a predetermined range, there is a case where it is better not to perform the adjusting operation in the decoder.
As mentioned above, upon automatization of the adjustment of the number of sampling data for the purpose of obtaining the stable image, for example, with respect to the sum of the differences of every field, there is a proper adjusting range which shows a criterion about whether the adjusting operation should be performed or not. The adjustment in a range out of the adjusting range contrarily enhances instability of the image.